Carboxylic acid esters are important chemical products, and have great solubility, fluidity, non-toxicity and photo-stability. The carboxylic acid ester may be used as materials for organic syntheses applicable to a plasticizer, a solvent, a tobacco, a spice, an essence, a cosmetic, a medicine, a dye, a surfactant, a rubber, a plastic, a polyester resin, an adhesive, an artificial fiber or a thin film material.
A traditional carboxylic acid ester is formed from an esterification of a carboxylic acid and an alcohol catalyzed by heat or an acid. The esterification is mainly performed in a liquid phase, and after the reaction, the product is neutralized by an alkali, washed and dried to remove sulfuric acid and byproducts, so as to obtain the carboxylic acid ester. Since the carboxylic acid ester is formed from the esterification of the alcohol and the carboxylic acid in the liquid phase, the esterification is limited by the reversible equilibrium, the reaction is incomplete and thus the conversion rate of the esterification is low. In order to increase the conversion rate of the esterification, concentrated sulfuric acid is commonly used as the catalyst for the esterification. However, the concentrated sulfuric acid is a strong oxidant and a strong dehydrating agent, and thus results in side reactions such as sulfonation, oxidation, etherification or carbonization, so as to adversely affect the recovery and quality of the ester compound. Further, due to using the concentrated sulfuric acid as the catalyst, the anti-corrosion equipment is needed and increases the production cost. Moreover, after the reaction, a lot of acidic waste liquid would impair environment.
Currently, to increase the conversion rate of the esterification, Chinese Patent No. 1332924C discloses an esterification of an organic acid. The conversion rate of this esterification is more than 95%, but has the concentrated sulfuric acid as a catalyst. The acidic waste liquid would result in corrosion to equipment and in environmental pollution. Therefore, this method is not suitable for continuous productions in the industry.
U.S. Pat. No. 5,536,856 discloses an esterification method and a device for preparing a carboxylic acid ester. In this method, an ionic resin having sulphonic acid groups and/or carboxylic acid groups are used as a catalyst and stacked in a plurality of trays. The esterification is performed at a predetermined pressure, wherein the reaction of the alcohol vapor and the acid is performed via the catalyst. This method consumes a lot of alcohol and carboxylic acid, but the carboxylic acid cannot be completely conversed. Therefore, the production cost is high in this method.
Chinese Patent Application Publication No. 100457263C discloses a method for preparing a catalyst of a hetero polyacid and/or a hetero polyacid salt carried on an inorganic carrier. The catalyst is used for preparing a low-carbon carboxylic acid ester. The catalyst may reduce corrosion to equipment and environmental pollution. However, the preparation of the catalyst is complicated, the activity period of the catalyst is short, and the esterification rate is less than 90%.
Chinese Patent Application Publication No. 100503043C discloses an ionic liquid catalyst for an esterification, the preparation method and the use thereof. The ionic liquid catalyst has high reactivity and causes less environmental pollution. However, the reaction retention time is about 2 to 12 hours, and after the reaction, the ester compound is separated from the ionic liquid catalyst by gravity sedimentation. Therefore, the esterification and the purification of the ester product are time-consuming.
Hence, the sulfuric acid is used as the catalyst increasing the esterification rate, but causes corrosion to equipment and environmental pollution in the prior art.
Accordingly, there is a need to develop a method for continuously preparing a carboxylic acid ester with a high esterification rate, high selectivity and simple purification.